Vibrations in rolling wheel assemblies (tyre+rim) are caused by several, different types of tyre imperfections, the most important being:                Non-homogeneous mass distribution        Stiffness variations        Geometric variations        Radial and lateral run outs        Eccentricity        
Similarly, imperfections in the rim will also induce vibrations.
The at present most common method for elimination of wheel assembly derived vibrations is still the attachment of lead weights to the tyre rim. However, quite apart from giving rise to a diffuse lead pollution in the environment through the weights falling off, this method can only compensate (and even at best only partially) for vibrations caused by non-homogeneous mass distribution in the wheel assembly, and since the other imperfections mentioned above are just as important sources of vibrations, lead weight balancing is not a satisfactory method for balancing of modern quality tyres.
European Patent No. 0557365 describes a different approach to reduction of vibrations induced by wheel assembly imperfections, comprising introduction of a visco-plastic gel into the tyre cavity. The principle behind the action of such gels is that they are able to flow under the stresses induced by vibrations and therefore spontaneously distribute themselves in a rolling tyre in such a way that the vibrations are reduced irrespective of what type of imperfections they are induced by. The efficiency of these balancing gels as compared to lead weight balancing can be demonstrated experimentally by measuring vertical accelerations on the spring leg of a front axle with its wheels running under load against a rotating drum. Typical results have shown that lead weight balancing actually increases vertical accelerations which clearly demonstrates that other sources of vibrations than non-homogeneous mass distribution are more important in this case. However, such balancing gels cannot completely eliminate vibrations caused by wheel assembly imperfections, because the centrifugal stress induced in a balancing gel by localised thickening (which occurs to diminish vibrations) will eventually be as strong as the residual vibrational forces caused by the imperfections, and therefore an equilibrium is established where a certain level of vertical accelerations will remain.
Also, a drawback with gel compositions in general is that a fairly large amount of gel must employed per tyre since a gel is not able to form localized “lumps” in the manner of a lead weight but must instead form a film layer with a continuously and gradually changing depth in order to provide the required balancing.